1. Field of the Invention
The present invention relates to surface-modified colour pigments and methods for preparing them. The present invention also relates to non-aqueous pigment dispersions using these pigments
2. Description of the Related Art
Polymeric dispersants are substances for promoting the formation and stabilization of a dispersion of pigment particles in a dispersion medium. They typically contain in one part of the molecule so-called anchor groups, which adsorb onto the pigments to be dispersed. In a spatially separate part of the molecule, polymeric dispersants have polymer chains compatible with the dispersion medium, thus stabilizing the pigment particles in the dispersion medium.
In aqueous pigment dispersions, the polymeric dispersants generally contain hydrophobic anchor groups exhibiting a high affinity for the pigment surface and hydrophilic polymer chains for stabilizing the pigments in the aqueous dispersion medium.
Thermally stable non-aqueous dispersions with submicron particles are much more difficult to prepare, especially for pigments having a non-polar surface. Particularly high requirements are set when manufacturing solvent based, oil based and radiation curable inkjet inks. Inadequate dispersing manifests itself as increased viscosity in liquid systems, loss of brilliance and/or hue shifts. Moreover, particularly good dispersion of the pigment particles is required to ensure unimpeded passage of pigment particles through the nozzles of a print head. In addition, pigment particle agglomeration and the associated blockage of the printer nozzles have to be avoided in the standby periods of the printer.
One approach to disperse pigments with a hydrophobic surface in a non-aqueous dispersion medium is by modifying the pigment surface to a hydrophilic surface by addition of so-called dispersion synergists. WO 2007/060254 (AGFA GRAPHICS) discloses quinacridone derivatives having carboxylic acid groups for modifying the surface of quinacridone pigments.
EP 430875 (CIBA GEIGY) discloses water-based coating materials containing a quinacridone pigment which is mixed with its sulphonated derivative (see example 11-13).
EP 504922 (HOECHST) discloses dioxazine pigments in acrylic resin lacquers wherein the surface of the pigment is modified by coating with sulphonic acid group-containing dioxazine derivatives.
EP 851005 (SEIKO EPSON) discloses in example 2 the preparation of a pigment including a quinacridone to which a sulfonic acid group is attached.
EP 894835 (TOYO INK) discloses an inkjet recording liquid comprising an organic pigment in particulate form, wherein the interior of each pigment particle particle is substantially free of sulfonic acid groups and the surface of each pigment particles carries a sulfonic acid group.
JP 2001-081354 (KONICA) discloses inkjet inks containing a coloring material comprising a compound containing a dye residue with a nitrogen atom bonded to a sulfonic acid or sulfonate group.
In another approach, the pigment surface is modified by direct chemical reaction. WO 2008/034472 (AGFA GRAPHICS) discloses a surface modification of several yellow pigments by hydrolyzing their methyl ester groups though the addition of sulfuric acid.
EP 1678264 A (FUJIFILM) discloses non-aqueous, radiation-curable inkjet inks comprising pigment, dispersant and a liquid carrier, wherein the pigment has an acid value greater than 8 mg of NaOH per gram of pigment. It is not disclosed how such pigments should be prepared.
US 2003134938 (DAINICHISEIKA) discloses a process producing a sulfonated solid particle by burning sulfur to yield gaseous sulfur dioxide, subjecting the gaseous sulfur dioxide to catalytic oxidation to yield gaseous sulfur trioxide, and sulfonating a dry powdery or granular solid particle with the gaseous sulfur trioxide in a gas phase-solid phase reaction. Sulfur dioxide and sulfur trioxide are highly toxic compounds which can also cause chemical burn on the skin of operators. Their use is to be avoided from a safety and also an environmental perspective. Furthermore, sulfonation using gaseous sulfur trioxide tends to be inhomogeneous, whereby not all particles are sufficiently sulfonated or pigment surfaces are only very locally sulfonated, resulting in poor dispersion quality and stability. Even when using a fluid bed process to avoid the generation of preferential flow through channels for the gaseous sulfur trioxide, the dispersion quality and stability could not be maximized. In addition, very small amounts of water vapour present during sulfonation lead to the formation of sulfuric acid, which must be neutralized and/or washed away to avoid dispersion problems.
Although several approaches exist for enhancing dispersion quality and stability of non-aqueous pigment dispersions, it remains difficult to obtain this for a number of pigments, such as e.g. C.I. Pigment Violet 23, a pigment of large interest for automotive lacquers.
Therefore, it is highly desirable to obtain via a simple synthesis, having minimal health and environmental impacts, modified pigments which can be dispersed in non-aqueous media to exhibit high dispersion quality and stability.